The immune system is a high complex network of interacting cells, receptors and factors that combine to yield a potent immune response. aver the last decade, major advances in the study of the immune system had depended on our ability to examine individual cells of the immune network. Automated flow microfluorometry using fluorescence activated cell sorters has enabled immunologists to analyze cell surface as well as intracellular proteins and DNA. The instruments allows detailed analysis of subpopulations represented by as few as 1% of the cells. It allows highly efficient preparative separations of individual cell populations and can analyze intracellular events such as calcium fluxes. All of the members of the program project need and use the cell sorter facility. Each of the projects depends on the multifaceted capabilities of these instruments to assure completion of the proposed studies. The cell sorter facility is, therefore, an essential resource for the research proposed in the program project.